the opposite ear. Low-frequency pure tones slightly increase the loudness of speech, but highfrequency tones produce no noticeable effect. DISCUSSION One explanation of the increase in loudness of speech when noise appears in the opposite ear would maintain that the noise has an effect upon the muscles of the contralateral middle ear. Presumably, this action of the middle-ear muscles would be such as to increase the physical intensity of the speech received at the inner ear. This view is in agreement with the slow decline in the loudness of speech back to its "normal" value after the noise is turned off. Such a view would leave to some other factor the change in localization of speech as noise is turned up in one ear. There is another plausible interpretation of this phenomenon. When identical stimuli fall on the two ears the total impression of loudness is greater than that aroused by either ear alone.Under these conditions the listener cannot "hear out" the two components, one from each ear, and then assess the loudness of each component. If the stimuli are markedly different, however, the loudness from on/e ear may not sum with the loudness from the bther ear. 4 Thus, if one pure tone is led to one ear and a pure tone of a different frequency is led to the other ear, the amount of summation of loudness apparently depends upon the similarity in frequency between the two tones. It is probable, therefore, that a noise in one ear increases the loudness of Speech heard in the other ear because of the similarity between the temporal and frequency characteristics of thermal noise and speech. In the discrimination of loudness the listener simply cannot distinguish between the loudness of one component as against the other component. Some experiments have been conducted to determine the effect of intense high frequency airborne sound on mice and a variety of insects. The sound source was a high frequency siren. The frequency used was about 20 kc and its acoustic level, in the region where the subjects were placed, was between 160 and 165 db (relative to 10 -16 watts/cm"). With sufficient exposure--from 10 seconds for flies and mosquitoes to 3 or 4 minutes for roaches and caterpillars--the sound proved lethal in all cases. More detailed work was performed on mice and the roach, Periplaneta americana. In both cases it was definitely established that the heating prodriced by sound absorption was sufficient to cause death. In addition to the heating there are other effects, notably tissue rupture, as evidenced by the almost complete destruction of the wings on flies and mosquitoes and the rapid deterioration and final disappearance of the external pinna of a mouse which had received a sub-lethal dose. During observations it has been impossible to completely avoid personal exposure to the sound field and some of the effects observed under these conditions will be described. These include momentary dizziness, and heating of exposed parts of the hand. HERE is considerable literature on the biological effects of intense liq...
The nonlinear resistance of a small, sharp-edged orifice has been used for over twenty years to provide a level-dependent noise reduction in an earplug. It serves well in applications for which protection is required against high-level momentary impulses, such as gunfire, and when low noise reduction is needed between impulses for improved reception of speech and other low-level signals. An earmuff has been developed with similar performance characteristics, but designed to serve a wider range of applications. Its attenuation rises instantaneously for impulses above 120 dB, and approaches that of a conventional earmuff for impulses that reach 160 dB. Between or in the absence of impulses, it allows good reception of speech and similar signals by providing attenuation that is substantially fiat from 400 to 8000 Hz and as much as 15 dB less than that of conventional earmuffs in this frequency range. Sufficient attenuation is retained to provide adequate hearing protection when the earmuff is used in the continuous moderate-level background noise encountered in many working environments. This paper describes the development of this earmuff and presents performance data illustrating the benefits realized with its new design.
This paper describes the design, construction and performance of a high intensity, high frequency (3–34 kc) siren and some rather striking phenomena which occur in the intense sound field produced by it. The siren is of the usual type consisting of a rotor which interrupts the flow of air through orifices in a stator. The rotor consists of a disk, approximately 6 in. in diameter, with 100 equally spaced slots and is driven by a small 23s hp high speed motor whose speed is varied by changing the applied voltage. The 100 corresponding holes in the stator are circular in section. For better radiation at the lower frequencies a plywood horn is mounted on the siren. The siren itself is small, and may be operated in any orientation. In its initial form, operating with chamber pressures in the neighborhood of 0.2 atmosphere, its measured efficiency was between 17 percent and 34 percent in the frequency range 3 to 19 kc with an acoustic output between 84 and 176 watts. There is a fair amount of parasitic noise, but it is negligible compared to the signal. With recent modifications, chamber pressures of about 2 atmospheres were obtained, yielding acoustic outputs of approximately 2 kilowatts, and an efficiency of about 20 percent.
The influence of turbulent flow on the sound attenuation in a narrow rectangular duct is calculated by a finite difference approximation of the differential equation for inviscid flow. The absorptive lining at one duct wall is characterized by a flow-independent wall impedance. Results are found in good agreement with experimental data from a duct with 1-in.2 open area lined on one side with a resistive layer of very fine metal fibers in front of a partitioned air backing. For resistive layers of fibers with larger diameter or of perforated plate used in the same configuration, the wall impedance depends greatly upon the flow velocity. A correlation is found between the nonlinearity of resistive layers due to turbulent flow and due to high sound-pressure levels in a duct. The influence of the nonlinearity due to high sound-pressure levels up to 160 dB is studied in the absence of flow. Measured attenuation data agree with those from a theoretical model, which correlates the varying attenuation along the duct with the varying sound-pressure difference across the resistive lining. By comparison of theoretical and experimental results, the influence of nonlinearity due to turbulent flow can be described by an equivalent level of sound-pressure difference across the resistive lining. A biasing effect of flow on nonlinear duct lining material linearizes the attenuation along the duct, if that equivalent level due to flow exceeds the actual sound level.
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